Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference herein, in its entirety.
The human complement system is a powerful player in the defense against pathogenic organisms and the mediation of immune responses. Complement can be activated through three different pathways: the classical, lectin, and alternative pathways. The major activation event that is shared by all three pathways is the proteolytic cleavage of the central protein of the complement system, C3, into its activation products C3a and C3b by C3 convertases. Generation of these fragments leads to the opsonization of pathogenic cells by C3b and iC3b, a process that renders them susceptible to phagocytosis or clearance, and to the activation of immune cells through an interaction with complement receptors (Markiewski & Lambris, 2007, Am J Pathol 171: 715-727). Deposition of C3b on target cells also induces the formation of new convertase complexes and thereby initiates a self-amplification loop.
An ensemble of plasma and cell surface-bound proteins carefully regulates complement activation to prevent host cells from self-attack by the complement cascade. However, excessive activation or inappropriate regulation of complement can lead to a number of pathologic conditions, ranging from autoimmune to inflammatory diseases (Holers, 2003, Clin Immunol 107: 140-51; Markiewski & Lambris, 2007, supra; Ricklin & Lambris, 2007, Nat Biotechnol 25: 1265-75; Sahu et al., 2000, J Immunol 165: 2491-9). The development of therapeutic complement inhibitors is therefore highly desirable. In this context, C3 and C3b have emerged as promising targets because their central role in the cascade allows for the simultaneous inhibition of the initiation, amplification, and downstream activation of complement (Ricklin & Lambris, 2007, supra).
Compstatin was the first non-host-derived complement inhibitor that was shown to be capable of blocking all three activation pathways (Sahu et al., 1996, J Immunol 157: 884-91; U.S. Pat. No. 6,319,897). This cyclic tridecapeptide binds to both C3 and C3b and prevents the cleavage of native C3 by the C3 convertases. Its high inhibitory efficacy was confirmed by a series of studies using experimental models that pointed to its potential as a therapeutic agent (Fiane et al., 1999a, Xenotransplantation 6: 52-65; Fiane et al., 1999b, Transplant Proc 31:934-935; Nilsson et al., 1998 Blood 92: 1661-1667; Ricklin & Lambris, 2008, Adv Exp Med Biol 632: 273-292; Schmidt et al., 2003, J Biomed Mater Res A 66: 491-499; Soulika et al., 2000, Clin Immunol 96: 212-221). Progressive optimization of compstatin has yielded analogs with improved activity (Ricklin & Lambris, 2008, supra; WO2004/026328; WO2007/062249). One of these analogs is currently being tested in clinical trials for the treatment of age-related macular degeneration (AMD), the leading cause of blindness in elderly patients in industrialized nations (Coleman et al., 2008, Lancet 372: 1835-1845; Ricklin & Lambris, 2008, supra). In view of its therapeutic potential in AMD and other diseases, further optimization of compstatin to achieve an even greater efficacy is of considerable importance.
Earlier structure-activity studies have identified the cyclic nature of the compstatin peptide and the presence of both a β-turn and hydrophobic cluster as key features of the molecule (Morikis et al., 1998, Protein Sci 7: 619-627; WO99/13899; Morikis et al., 2002, J Biol Chem 277:14942-14953; Ricklin & Lambris, 2008, supra). Hydrophobic residues at positions 4 and 7 were found to be of particular importance, and their modification with unnatural amino acids generated an analog with 264-fold improved activity over the original compstatin peptide (Katragadda et al., 2006, J Med Chem 49: 4616-4622; WO2007/062249).
While previous optimization steps have been based on combinatorial screening studies, solution structures, and computational models (Chiu et al., 2008, Chem Biol Drug Des 72: 249-256; Mulakala et al., 2007, Bioorg Med Chem 15: 1638-1644; Ricklin & Lambris, 2008, supra), the recent publication of a co-crystal structure of compstatin complexed with the complement fragment C3c (Janssen et al., 2007, J Biol Chem 282: 29241-29247; WO2008/153963) represents an important milestone for initiating rational optimization. The crystal structure revealed a shallow binding site at the interface of macroglobulin (MG) domains 4 and 5 of C3c and showed that 9 of the 13 amino acids were directly involved in the binding, either through hydrogen bonds or hydrophobic effects. As compared to the structure of the compstatin peptide in solution (Morikis et al., 1998, supra), the bound form of compstatin experienced a conformational change, with a shift in the location of the β-turn from residues 5-8 to 8-11 (Janssen et al., 2007, supra; WO2008/153963).
In view of the foregoing, it is clear that the development of modified compstatin peptides or mimetics with even greater activity would constitute a significant advance in the art.